Abstract: Improved apparatus for injecting vapors, such as water vapor in the form of low temperature-low pressure steam, into the intake manifold of an internal combustion engine. A controlled flow of air is drawn through a vapor generator by the intake manifold vacuum and the resulting air-vapor mixture is discharged through a nozzle into the intake manifold. The vapor generator contains a level controlled body of liquid for forming the vapor. Air enters the generator at the bottom of the liquid and rises through the liquid past a heater and a dispersion member which disperses the air into small bubbles. The heater is operated from the engine cooling system and heats the air and liquid sufficiently for generating steam at the subatmospheric pressures available from the intake manifold. The resulting steam-air mixture is injected into the intake manifold for controlling combustion. In a modified embodiment, liquid is also injected directly into an engine carburetor during acceleration.